1. Field of the Invention
The present invention relates to a semiconductor laser, a method of manufacturing a semiconductor laser, an optical pickup and an optical disk apparatus, and is particularly suitable for application, for example, to a self-pulsation (self-oscillation) type semiconductor laser using a nitride type Group III-V compound semiconductor and to an optical disk using the semiconductor laser as a light source.
2. Description of the Related Art
In a high-density optical disk system, a GaN-based semiconductor laser in a 400 nm wavelength band is used as a light source. In this case, it may be necessary to reduce the return light noise of the GaN-based semiconductor laser. One of the measures to cope with this problem is a technique of causing a self-pulsation operation.
In order to realize such a self-pulsation operation, there has been proposed a method in which a saturable absorbing layer is provided at a light guide layer (guide layer) or a clad layer, and the saturable absorbing layer is doped with an impurity (refer to, for example, Japanese Patent Laid-Open No. Hei 9-191160). In relation to this system, however, it has been pointed out as a problem the instability of the self-pulsation operation against temperature (refer to, for example, Japanese Patent Laid-Open No. 2003-31898). It has also been proposed to enable a self-pulsation operation by selecting the thickness of a saturable absorbing layer, the light confinement coefficient, the distance between an active layer and the saturable absorbing layer, etc. or by inserting a wide-gap semiconductor between the active layer and the saturable absorbing layer (refer to, for example, Japanese Patent Laid-Open No. 2003-31898 and Japanese Patent Laid-Open No. 2003-218458). However, even the use of these conditions may not stably provide a semiconductor laser capable of a self-pulsation operation. For example, it has been reported that mere introduction of a p-type GaN layer between a saturable absorbing layer and a p-type AlGaN layer made the self-pulsation operation hard (refer to, for example, Appl. Phys. Lett., Vol. 83, p. 1098 (2003), herein after referred to as Non-Patent Document 1). Non-Patent Document 1 gives a conclusion that the promotion of recombination of carriers at the interface between the p-type AlGaN layer and the saturable absorbing layer and the tunneling of carriers due to a piezo effect make the carrier lifetime in the saturable absorbing layer shorter on an effective basis, thereby contributing to the self-pulsation operation. In any way, such instability forms a serious problem in the mass production of a GaN-based semiconductor laser designed to perform a self-pulsation operation.